I have always used flat MM grays as a base coat for my gloss or semi-gloss acrylics. Never had a problem with paint lifting or incompatablility. I just let the enamel cure for a few days before applying it. [:)]
It’s the glossiness that causes the acrylic not to stick well. Part of the gloss in a glossy enamel paint is due to the resin or polymer used, but part may also come from a silicone additive which gives the paint a very high gloss and adds to water repelancy. Flat paints do not contain the silicone additive, and have more surface relief, or “tooth” to form a mechanical bond with the acrylics.
So, no, you are probably not screwed for your Do 335. If your enamel is a model paint, you are probably in good shape. If, however, it is not a model paint, it may have been made with a resin that is hydrophobic. If you do have adhesion problems due to this, a light sanding should solve the problem.
Bear in mind that freshly cured (completely hardened) enamels continue to outgas for a significant time after full cure. Allow the enamel to sit for a few days before applying acrylic.